To increase production of a desired hydrocarbonaceous fluid from a subterranean formation about a well, it is frequently necessary to increase the permeability. One of the ways that this can be accomplished is by hydraulic fracturing, which is the process of injecting a viscous fluid at sufficiently high rates and pressures that the formation must fracture to accept the fluid. To keep the fracture from fully closing after the fluid injection has ceased, it is necessary to deposit a proppant with the fracture fluid. One of the difficulties of the earlier art is that the proppants have tended to be produced back into the well causing the fracture to close and have their permeability reduced by crushing.
It is desirable to consolidate the proppant particles together and to have a relatively high compressive strength so as to resist crushing of the proppant under the overburden pressure. In this way, the proppant is retained in the formation, and is not produced back into the bore hole. Resin-coated proppants consolidated downhole have been used to offset this problem.
Any resin-coated proppant should be compatible with the fracturing fluids used to carry it downhole and be as economical as possible. Moreover, extra steps should be curtailed when feasible and eliminated where possible, as will be discussed in more detail hereinafter.
The prior art has ranged from U.S. Pat. No. 3,851,479 showing sealed porous earth formations where the void spaces are reduced with the treatment with aqueous solutions containing hydroxl ions, polyisocyanurates, and polyvinyl alcohol, through U.S. Pat. No. 4,114,382 concerned with consolidating geological formations with a polyol and a special 1,2 alkylene oxide, to recent patents; such as U.S. Pat. No. 4,709,002 describing a method for preparing a molded reaction injection rigid non-cellular polyisocyanurate and U.S. Pat. No. 4,731,427 describing a method of preparing a molded reaction injection rigid polyurethane modified isocyanurate polymer composition. While these patents are pertinent chemically in that the reactants initially are quite similar, the result is vastly different from the invention herein. Specifically, it is desirable that the method of coating a proppant have the following features as well as those earlier noted and not heretofore provided:
1. A problem encountered during experimentation has been that the shearing of the resin-coated proppant causes the resin to come off the proppant and results in little or no consolidation. Therefore a more viscous, high molecular weight resin has been employed in this invention, which has a lower energy of activation.
2. Another problem has been that, when simulating shearing action that the coated proppant, or sand, will undergo before or during pumping downhole, the turbulent motion should not cause premature polymerization of the resin. Expressed otherwise, polymerization should not be occurring at this time. It is desirable that a reduced degree of reactivity of any coating be achieved so that it does not come off the sand when admixed with a carrier at this time.
3. The quasi prepolymer allows the use of a slowacting catalyst such as water if desired.
4. The resin should have a higher viscosity and a greater wettability on the proppant, than provided heretofore.
5. As indicate hereinbefore, there should be elimination of the step of providing a silane coupling reagent as is done with other resin-coated proppants.
6. The resin coating the proppant should be easier to use in the field. Expressed otherwise, it should exist as a single solution, or one component polyurethane system, that can be coated onto the proppant readily.
7. Also, the prepolymer should stay coated onto the proppant and not defuse off into the formation. In our invention, it does not, and therefore the pore spaces stay open and are not filled as in the aforementioned patents and inventions.
It is apparent that the prior art has failed to provide the above delineated features.